Hey there, I was on the depo shot for about 2-3 years.
I experienced the same thing when I got off.
I was not on any type of birth control for nearly a year and
i did not have my period at all.

I was worried as well and I went to my doctor. She told me it was
normal and was an after effect since my body was so used to the
effects of the depo shot. If I wanted to, she could prescribe me
some type of estrogen pill to regulate my body.

I tookd a pregnancy test and then I got back on the depo shot
and my period came again.

To be on safe side, you should talk to your doctor about
it but you shouldn't be too worried about it b/c your body
hasn't adjusted to the new changes or just isn't back to normal yet.

I do suggest you go back on some sort of birth control if you
are sexually active. =) well good luck!

- l